feat(P01-05-01,P01-05-02,P01-05-03): coaching debrief + guardrail filter + TTS voice

server/debrief.py — generate_debrief() loads the session turns + branch
outcome + scenario debrief.debrief_focus, calls deepseek-v4-flash:cloud in
no_think mode (D-020, REQ-LLM-02) with the debrief prompt template
(docs/debrief/default.yaml), produces a concise 3-bullet text summary
(what you did well / what to improve / one next step) referencing the
learner's actual turns + branch outcome.

TASK-05-02: the debrief text is routed through the CustomerServiceGuardrail
output filter (debrief role) — legal-action recommendations are blocked and
replaced with a coaching redirect.

TASK-05-03: the debrief is synthesized via the same TTSProvider interface
as the role-play (D-006 one voice) — no separate TTS path (verified by
structural test).

5 tests pass (debrief references turns + branch, no_think mode asserted,
guardrail blocks 'tell the customer to sue', normal coaching passes,
TTSProvider synthesis reuses role-play voice).

---ci---
phase: 1
milestone: v0.1
plan: 05
task: 05-01,05-02,05-03
status: execute
persona: backend-engineer
requirements:
  covered: [REQ-DEBRIEF-01, REQ-LLM-02, REQ-NFR-SAFE-01]
---/ci---

docs/debrief/default.yaml

@@ -0,0 +1,26 @@
+# Default debrief prompt template (TASK-05-01).
+# Renders the learner's turns + branch outcome + debrief_focus into a coaching prompt.
+# Uses deepseek-v4-flash:cloud no_think mode (D-020) for latency.
+
+system: |
+  You are a coaching mentor for a customer-service role-play training session.
+  Produce a concise (3-bullet) debrief about the learner's performance.
+  Structure:
+    - What you did well
+    - What to improve
+    - One next step
+  Base your feedback on the learner's ACTUAL turns (quoted below) and the
+  branch outcome. Do NOT reason step-by-step; respond directly (no_think).
+  Keep it about the learner's communication performance, not about the
+  customer's legal rights. Do not recommend that the learner advise a real
+  customer to take legal action.
+
+user: |
+  Scenario: {{ scenario_title }}
+  Branch outcome: {{ outcome }} ({{ branch_id }})
+  Debrief focus: {{ debrief_focus }}
+
+  Learner turns:
+  {{ learner_turns }}
+
+  Produce the 3-bullet debrief now.